Gas distribution plate electrode for a plasma reactor

ABSTRACT

The invention is embodied in a plasma reactor for processing a semiconductor wafer, the reactor having a gas distribution plate including a front plate in the chamber and a back plate on an external side of the front plate, the gas distribution plate comprising a gas manifold adjacent the back plate, the back and front plates bonded together and forming an assembly. The assembly includes an array of holes through the front plate and communicating with the chamber, at least one gas flow-controlling orifice through the back plate and communicating between the manifold and at least one of the holes, the orifice having a diameter that determines gas flow rate to the at least one hole. In addition, an array of pucks is at least generally congruent with the array of holes and disposed within respective ones of the holes to define annular gas passages for gas flow through the front plate into the chamber, each of the annular gas passages being non-aligned with the orifice.

BACKGROUND OF THE INVENTION

[0001] Various types of plasma reactors employed in the manufacture ofsemiconductor microelectronic circuits require a large RF electrode atthe reactor chamber ceiling that overlies the semiconductor workpiece.Typically, the workpiece is a semiconductor wafer supported on aconductive pedestal. RF power is applied to the support pedestal, andthe ceiling or overhead electrode is a counter electrode. In somereactors, the RF power applied to the support pedestal is the plasmasource power (determining plasma ion density) and is also the plasmabias power (determining ion energy at the wafer surface). In otherreactors, an RF power applicator other than the wafer pedestal furnishesthe plasma source power, while the RF power applied to the waferpedestal serves only as plasma RF bias power. For example, the plasmasource power may be applied by an inductive antenna or may be applied bythe ceiling electrode. Thus, the ceiling electrode may either be agrounded counter electrode for the RF power applied to the wafer supportpedestal or it may be connected to an independent RF power generator andfunction as an independent RF power applicator. In either case, the mostuniform distribution of process gas is obtained by introducing theprocess gas through the ceiling. This requires that the ceilingelectrode be a gas distribution plate.

[0002] There is a continuing need to improve the uniformity of processgas distribution across the wafer surface in a plasma reactor,particularly in a plasma reactor used for semiconductor etch processesas well as other semiconductor processes. This need arises from theever-decreasing device geometries of microelectronic circuits andminimum feature sizes, some approaching 0.15 microns. Such small devicegeometries are dictated in most cases by the desire for highermicroprocessor clock speeds, and require corresponding improvements inetch rates, etch uniformity across the wafer surface and damage-freeetching. Previously, with devices having relatively large feature sizes,a single gas inlet in the plasma reactor overhead ceiling electrode/gasdistribution plate provided adequate process gas distributionuniformity. A single inlet would necessarily be of a large size in orderto meet the requisite gas flow requirements.

[0003] One problem with such a large inlet is that it is moresusceptible to plasma entering the inlet and causing arcing or plasmalight-up within the inlet. Such arcing damages the plate and/or enlargesthe inlet and consumes power. Sputtering of the plate material aroundthe inlet can also contaminate the plasma with by-products of thesputtering. With a large hole, the maximum electric field occurs nearthe center of the hole, and this is the likliest location for plasmalight-up or arcing to begin. One solution proposed for reactors having asingle gas inlet was to juxtapose a disk or puck in the center of thehole to keep gases away from the intense electric field at the holecenter (U.S. Pat. No. 6,885,358 by Dan Maydan). However, with currentdevice geometries incorporating very small feature sizes, much betterprocess gas distribution uniformity across the wafer surface isrequired. As a result, a single gas distribution inlet or orifice in theceiling gas distribution plate is inadequate to provide the requisitegas distribution uniformity. Thus, an overhead gas distribution plate iscurrently made by drilling thousands of fine holes or orifices throughthe plate. The spatial distribution of such a large number of orificesimproves gas distribution uniformity across the wafer surface. Thesmaller size makes each hole less susceptible to plasma entering thehole.

[0004] Unfortunately, it has not been practical to place or hold anindividual puck in the center of each one of the thousands of holes toward the gas away from the high intensity electric fields near the holecenters. Thus, in order to reduce plasma arcing, the gas inlet holesmust be of minimal diameter and within a small dimensional hole-to-holetolerance to ensure uniform gas distribution. Drilling such a largenumber of holes is costly. This is because the holes must have such ahigh aspect ratio, must be drilled through very hard material (such assilicon carbide) and sharp hole edges must be avoided. Moreover, thevery need for such accurately sized holes means that performance iseasily degraded as hole sizes are enlarged by plasma sputtering of thehole edges. Depending upon plasma ion density distribution across theceiling surface, some holes will be widened at a greater rate than otherholes, so that a gas distribution plate initially having highly uniformgas distribution across the wafer surface eventually fails to providethe requisite uniformity.

[0005] Another problem is that the need for greater etch rate hasdictated a smaller wafer-to-ceiling gap in order to obtain denserplasma. The small gas orifices produce very high velocity gas streams.The high velocity gas streams thus produced can be so narrowlycollimated within the narrow wafer-to-ceiling gap that the hole-to-holespacing in the gas distribution plate produces corresponding peaks andvalleys in gas density at the wafer surface and correspondingnon-uniformities in etch rate across the wafer surface.

[0006] As a result, there is a need for an overhead gas distributionplate that functions as an electrode or counter electrode, and that isnot susceptible to plasma arcing in the gas injection passages, thatdoes not have high gas injection velocities and in which the gasdistribution uniformity and velocity are not affected by enlargement ofthe gas injection passages.

SUMMARY OF THE DISCLOSURE

[0007] The invention is embodied in a plasma reactor for processing asemiconductor wafer, the reactor having a gas distribution plateincluding a front plate in the chamber and a back plate on an externalside of the front plate, the gas distribution plate comprising a gasmanifold adjacent the back plate, the back and front plates bondedtogether and forming an assembly. The assembly includes an array ofholes through the front plate and communicating with the chamber, atleast one gas flow-controlling orifice through the back plate andcommunicating between the manifold and at least one of the holes, theorifice having a diameter that determines gas flow rate to the at leastone hole. In addition, an array of pucks is at least generally congruentwith the array of holes and disposed within respective ones of the holesto define annular gas passages for gas flow through the front plate intothe chamber, each of the annular gas passages being non-aligned with theorifice.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 is a simplified cut-away cross-sectional side view of aplasma reactor embodying the present invention.

[0009]FIG. 2A is a partially exploded cross-sectional side view of a gasdistribution plate of the plasma reactor of FIG. 1 in accordance with afirst embodiment.

[0010]FIG. 2B is a side view of an assembled gas distribution plate ofthe plasma reactor corresponding to FIG. 2A.

[0011]FIG. 3A is a plan view of one implementation of the front plate ofthe gas distribution plate of FIG. 2B.

[0012]FIG. 3B is a plan view of the front plate of FIG. 3A bonded to theback plate in accordance with this implementation.

[0013]FIG. 4 is a cross-sectional side view of the assembly of FIG. 3Bcorresponding to lines 4-4 of FIG. 3B.

[0014]FIG. 5 is a cross-sectional side view of a gas distribution plateof the plasma reactor of FIG. 1 in accordance with a second embodiment.

[0015]FIG. 6 is a cut-away partially exploded perspective view of a gasdistribution plate of the plasma reactor of FIG. 1 in accordance with athird embodiment.

[0016]FIG. 7 is a cross-sectional view corresponding to lines 7-7 ofFIG. 6.

[0017]FIGS. 8A, 8B, 8C and 8D are sequential cut-away partial side viewsof one portion of a gas distribution plate of FIG. 6, illustrating afirst process for fabricating the gas distribution plate of FIG. 6.

[0018]FIGS. 9A, 9B, 9C and 9D are sequential cut-away partial side viewsof one portion of a gas distribution plate of FIG. 6, illustrating asecond process for fabricating the gas distribution plate of FIG. 6.

[0019]FIG. 10 is a cross-sectional side view of a gas distribution plateof the plasma reactor of FIG. 1 in accordance with a third embodiment.

[0020]FIG. 11 is a cross-sectional side view of an alternate gasdistribution plate as shown in FIG. 10.

DETAILED DESCRIPTION

[0021] Referring to FIG. 1, a plasma reactor includes a vacuum chamber100 bounded by a reactor chamber cylindrical side wall 105, a ceiling110 and floor 115. A vacuum pump 120 maintains a vacuum within thechamber at a desired chamber pressure. A wafer support pedestal 125 forsupporting a semiconductor wafer or workpiece 130 is disposed at thebottom of the chamber 100 so that the wafer 130 faces the ceiling 110.The wafer support pedestal 125 has conductive elements so that thepedestal 125 can serve as an electrode or RF power applicator. For thispurpose, an RF generator 135 is connected to the pedestal 125 through anRF impedance match circuit 140. The ceiling 110 is conductive in theillustrated embodiment and is connected to the RF return terminal of theRF generator 135 so that the ceiling 110 serves as a counter electrodefor the wafer pedestal 125. In some types of reactors, another RFgenerator 145 may be connected to the ceiling 110 through an RFimpedance match circuit 150, so that the ceiling 110 also serves asanother RF power applicator. In this case, the frequencies of the two RFgenerators 135, 145 are very different so that the two RF generators135, 145 function independently.

[0022] Process gas is introduced so as to provide maximum gasdistribution uniformity across the top surface of the wafer 130 byinjecting it through many uniformly spaced gas injection inlets 160 inthe ceiling 110. The ceiling 110 is thus a gas distribution plate. A gassource or supply 165 is coupled to a gas manifold 170 in the ceiling/gasdistribution plate 110, and the gas manifold 170 feeds each of theinlets 160. As shown in FIGS. 2A and 2B, the inlets 160 of the gasdistribution plate 110 are formed by two parallel planar plates, namelya back plate 205 and a front plate 210 which are manufactured separately(FIG. 2A) and then bonded together (FIG. 2B). The back plate 205 is ontop and the front plate 210 is on the bottom and faces the plasma in theinterior of the chamber 100. The back plate 205 consists of an array ofrelatively large cylindrical openings 215 in its bottom surface whilethe front plate 210 consists of an array of cylindrical pucks 220matching the array of openings 215. As shown in FIG. 2B, the pucks 220of the front plate 210 fit within the openings 215 of the back plate205, the clearance between each opening 215 and matching puck 220forming an annular gap therebetween, the annular gap being the gas inlet160. Gas feed orifices 230 in the back plate 205 are sized to providethe precise gas flow desired extend vertically from the gas manifold 170overlying the back plate 205 to the annular gas inlets 160. Since thegas distribution plate 110 consists of an array of hundreds or thousandsof annular inlets 160 to achieve spatially uniform gas distributionacross the entire wafer surface, the inlets 160 would in most casesallow too much gas flow. Therefore, the finely-sized orifices 230provide the requisite flow control.

[0023] Significantly, each orifice 230 faces a horizontal gap 235between the respective puck 220 and the back plate 205, so that the gasis forced to make an abrupt turn to enter the gap 235 and another abruptturn to enter the annular inlet 160. It is difficult if not impossiblefor plasma in the chamber travelling upward in the annular inlets 160 tomake both of these turns without being extinguished by collisions withthe gas distribution plate surfaces within the annular inlet 160 and thehorizontal gap 235. A result is that the precisely sized orifices 230are protected from plasma sputtering. This leaves only the annularinlets 160 subject to distortion in size from plasma sputtering orattack. However, the area of each annular inlet 160 is so large thatplasma sputtering introduces only a small fractional difference in areafrom inlet to inlet, so that gas distribution uniformity across thewafer surface is virtually immune to such changes. Moreover, in theembodiment of FIGS. 2A and 2B, gas flow uniformity is determined by theuniformity of the orifices 230 only, so that changes in the sizes of thevarious annular inlets 160 have virtually no affect on gas flowuniformity. Thus, performance of the gas distribution plate 110 isvirtually immune to changes induced by plasma sputtering or attack, asignificant advantage.

[0024] In one embodiment, the back plate 205 and front plate 210 areformed of silicon carbide and are bonded together using existingtechniques in silicon carbide manufacturing. One advantage of usingsilicon carbide as the material of the gas distribution plate 110 isthat such material is practically impervious to attack by certainprocess gases and plasma species, such as halogen-containing processgases and plasma species. Also, silicon carbide is relatively compatiblewith silicon semiconductor wafer processing, so that contamination fromplasma sputtering of such material is not as harmful as are othermaterials such as aluminum.

[0025] Another advantage of the annular-shaped gas inlets 160 is thateach puck 220 keeps the plasma ions and gases away from the center ofeach opening 215 where electric fields are maximum. This feature helpsprevent arcing or plasma light-up. The two-plate structure 205, 210 ofthe gas distribution plate 110 enables cost-effective manufacture ofhundreds or thousands of holes 215 and pucks 220 centered in each of theholes. The invention thus provides an economical gas distribution platewith sufficient uniformity of gas distribution to process extremely finedevice features (e.g., 0.15 microns) on a very large wafer (10 inch to20 inch diameter) with minimal plasma arcing while being impervious tolong-term wear from plasma sputtering.

[0026] Another advantage is that the relatively large annular openings160 provide a much lower gas injection velocity. Although each finelysized orifice 230 produces a very high velocity gas stream into therespective horizontal gap 235, passage through the horizontal gap 235and through the large annular inlet 160 dissipates its velocity. As aresult, the gas flow from the bottom of the front plate 210 is much moreuniform and free from high velocity narrow gas streams and plasmaplumes. Therefore, a small wafer-to-ceiling gap does not lead to spatialnon-uniformities in the gas distribution at the wafer surface using thegas distribution plate 110, a significant advantage.

[0027] Many of the advantages enumerated above are pertinent to problemsencountered in high power plasma reactors capable of high plasma iondensities. One of these problems is that high plasma ion density overthe wafer surface is achieved in some reactors by a smallwafer-to-ceiling gap to better confine the plasma. As noted above, thegas distribution plate 110 provides uniform gas distribution within sucha small gap because of the large size of the annular inlets 160. Anotherone of these problems is that high plasma ion density is achieved insome reactors by applying plasma source power to the ceiling or overheadgas distribution plate, which leads to arcing in the gas inlets. Asnoted above, the gas distribution plate 110 includes the pucks 220 thatconfine the gas closer to the periphery of each hole 215 where electricfields are minimum so as to suppress or prevent arcing. Thus, the gasdistribution plate 110 is inherently suitable for use in high densityplasma reactors.

[0028]FIGS. 3A, 3B and 4 illustrate one implementation of the embodimentof FIGS. 2A and 2B. FIG. 3A shows that the front plate 210 having thearray of pucks 220 consists of a web of longitudinal arms 310 andlateral arms 315 formed with the pucks 220 and holding them in the fixedarray. Referring to FIGS. 3B and 4, the back plate 205 has longitudinalchannels 320 and lateral channels 325 that receive the longitudinal andlateral arms 310, 315 when the plates 205, 210 are joined together. Thepucks 220 are centered in the respective holes 215 and spaced apart fromthe back plate 205 by the horizontal gaps 235 and the annular inlets 160and therefore do not contact the back plate 205. Contact between theback plate 205 and the front plate 210 is along the longitudinal andlateral arms 310, 315 that fit snuggly within the correspondinglongitudinal and lateral channels 320, 325. It is along these contactingsurfaces that the two plates 205, 210 are bonded together. As notedpreviously above, if the two plates are silicon carbide material, thenthe bonding is carried out using standard silicon carbide bondingtechniques.

[0029]FIG. 5 illustrates an embodiment in which a single orifice 235 afeeds a group of neighboring annular gas inlets 160 a, 160 b, 160 c. Thesingle orifice 235 a feed the middle annular gas inlet 160 b directlyvia the horizontal gap 235 b, and feeds the adjacent annular inlets 160a, 160 c through internal channels 505, 510 connecting the adjacentannular inlets 160 a, 160 c with the middle annular inlet 160 b. Oneadvantage of this embodiment is that the number of finely sized orifices235 that must be drilled in the back plate 205 is greatly reduced.

[0030]FIG. 6 illustrates an embodiment in which a back plate 600 hasparallel lateral slots 605 and a front plate 610 has an array of holes615 and pucks 620. The circular holes 615 and the cylindrical pucks 620are concentrically arranged so that they define corresponding annulargas ports 616. The slots 605 are aligned with respective rows of theholes 615 and pucks 620. The width of each slot 605 is less than thediameter of each hole 615 (e.g., less than half). The plates 600, 610are joined together so that each slot 610 is centered with a respectiverow of the array of holes 615. Referring to the cross-sectional view ofFIG. 7, the resulting gas passage aligned with each hole 615 consists ofa pair of arcuate slots 630 a, 630 b which appear in FIG. 7 in solidline. Process gas is fed into each slot 605 by a single fine orifice 635through the back plate 600. The diameter of the orifice 635 is selectedto provide the requisite gas flow rate.

[0031] The embodiment of FIGS. 6 and 7 is simpler to form because thereis no horizontal gap (e.g., the horizontal gap 235 of FIG. 2) betweenthe puck 620 and the back plate 600. Instead, the bond between theplates 600, 610 is formed along the entirety of their adjoiningsurfaces. The pucks 620 are similarly bonded across the entirety oftheir top surfaces to the bottom surface of the plate 600. The onlyareas of the top surfaces of the pucks 620 not thus bonded are the areasfacing the narrow slots 605.

[0032] In the foregoing embodiments, the pucks 620 function as flowdiversion elements for transforming gas flow between the front and backplates 610, 600 from stream patterns in the back plate 600 to annularflow patterns in the front plate 610. The stream patterns correspond toa first radius (i.e., the radius of the top orifices 635) and theannular patterns correspond to a second radius (i.e., the radius of eachannular opening 660) which is larger than the first radius. The flowdiversion elements 620 induce a rapid change of gas flow (a) from avertical flow of the stream pattern in each orifice 635 (b) to ahorizontal flow from the first radius (of each orifice 635) to thesecond radius (of the corresponding annular opening 660) and (c) to avertical flow in each corresponding annular opening 660.

[0033] FIGS. 8A-8D illustrate one method for fabricating the gasdistribution plate of FIGS. 6 and 7 as a monolithic silicon carbidepiece. In FIG. 8A, the back plate 600 is formed of sintered siliconcarbide and the slots 605 are milled in the plate 600. In FIG. 8B,graphite inserts 805 are placed in the slots 605. In FIG. 8C, the frontplate 610 is formed by chemical vapor deposition of silicon carbide onthe bottom surface 600 a of the back plate 600. Then, the graphiteinserts are all removed by heating the entire assembly until thegraphite material burns away, leaving the slots 605 empty, as shown inFIG. 8D. In FIG. 8D, an array of annular openings 660 are milledcompletely through the entire thickness of the front plate 610,corresponding to the holes 615 and pucks 620 illustrated in FIG. 6. FIG.8D also depicts the orifice 635, which may be milled during one of theforegoing steps.

[0034] FIGS. 9A-9D illustrate another method for fabricating the gasdistribution plate of FIGS. 6 and 7 as a monolithic silicon carbidepiece. In FIG. 9A, the back plate 600 is formed of sintered siliconcarbide and the slots 605 are milled in the plate 600. In addition, awide shallow channel 810 is formed in the back plate 600 centered alongand parallel to each slot 605. In FIG. 8B, silicon carbide inserts 815are placed in the wide shallow slots 810 in FIG. 8C, the front plate 610is formed by chemical vapor deposition of silicon carbide on the bottomsurface 600 a of the back plate 600. In FIG. 8D, an array of annularopenings 660 are milled completely through the combined thicknesses ofthe front plate 610 and the silicon carbide inserts 815, correspondingto the holes 615 and pucks 620 illustrated in FIG. 6.

[0035]FIG. 10 illustrates yet another embodiment in which the back plate600 and the front plate 610 are both formed of anodized aluminum. Theanodization produces an alumina thin film 600-1 on the back plate 600and an alumina thin film 610-1 on the front plate 610. The anodizationlayer protects the aluminum plates from the plasma.

[0036] While the invention has been described with reference toembodiments in which the ceiling gas distribution plate must function asan electrode (and therefore comprise conductive material), the gasdistribution plate of the invention is also well suited to applicationsin which the gas distribution plate does not function as an electrode.

[0037] In those embodiments in which the ceiling gas distribution platefunctions as an overhead electrode, it may consist of silicon carbide,as described above. If it is desired that the gas distribution platehave a resistivity less than that of silicon carbide (0.005-1.0 Ohm-cm),then each of the silicon carbide plates 600, 610 may be fabricated insuch a manner as to have a thin highly conductive graphite layers 910,920 running through the center of the plates and co-planar with therespective plate, as illustrated in FIG. 11. This is accomplished byforming each plate 600, 610 as a graphite plate. Each graphite plate ismachined to form the structural features described above with referenceto FIGS. 6 and 7. Then, each graphite plate 600, 610 is siliconizedusing conventional techniques. However, the siliconization process iscarried out only partially so as to siliconize the graphite plates to alimited depth beyond the external surface of the graphite. This leavesan interior portion of the graphite un-siliconized, corresponding to thegraphite layers 910, 920 enclosed within the silicon carbide plates 600,610. The graphite layers 910, 920 have a resistivity about one order ofmagnitude less than that of silicon carbide. Since the graphite layers910, 920 are completely enclosed in silicon carbide, they are protectedfrom the plasma.

[0038] While the gas distribution plate of FIGS. 2A and 2B has beendescribed as being formed of silicon carbide, it may, instead, be formedof silicin.

[0039] While the invention has been described in detail with referenceto preferred embodiments, it is understood that variations andmodifications thereof may be made without departing from the true spiritand scope of the invention.

What is claimed is:
 1. A plasma reactor for processing a semiconductorwafer, said reactor comprising: a vacuum chamber side wall; a wafersupport pedestal for supporting said semiconductor wafer in saidchamber; an RF power source coupled to said wafer support pedestal; aprocess gas source; a gas distribution plate at a ceiling location ofsaid chamber, said gas distribution plate comprising: a front plate insaid chamber and a back plate on an external side of said front plate,said gas distribution plate comprising a gas manifold adjacent said backplate, said back and front plates bonded together and forming anassembly comprising: (a) an array of holes through said front plate andcommunicating with said chamber, (b) at least one gas flow-controllingorifice through said back plate and communicating between said manifoldand at least one of said holes, said orifice having a diameter thatdetermines gas flow rate to the at least one hole; (c) an array of pucksat least generally congruent with said array of holes and disposedwithin respective ones of said holes to define annular gas passages forgas flow through said front plate into said chamber, each of saidannular gas passages being non-aligned with said orifice.
 2. The reactorof claim 1 wherein said assembly further comprises an array of gasflow-controlling orifices through said back plate communicating betweensaid manifold and corresponding ones of said array of holes.
 3. Thereactor of claim 2 wherein each of said orifices faces a correspondingone of said pucks, said assembly further comprising a planar gap betweeneach of said pucks a facing surface of said back plate, each planar gapcommunicating between the corresponding orifice and the correspondingannular gas passage.
 4. The reactor of claim 1 wherein said assemblyfurther comprises: one gas flow-controlling orifice through said backplate for each predetermined group of neighboring holes, said orificegenerally facing a center one of the holes of the corresponding group ofneighboring holes; internal gas passages connecting the center one ofthe holes with the other holes in said group of neighboring holes. 5.The reactor of claim 3 wherein said back plate comprises said array ofholes and said front plate comprises said array of pucks.
 6. The reactorof claim 5 wherein said front and back plates comprises silicon carbide.7. The reactor of claim 6 wherein at least one of said plates furthercomprises an enclosed planar layer of graphite extending parallel with aplane of said plate.
 8. The reactor of claim 5 wherein said front andback plates comprises anodized aluminum.
 9. The reactor of claim 5wherein said gas distribution plate comprises a ceiling electrode ofsaid reactor, said reactor further comprising a second RF power sourcecoupled to said gas distribution plate.
 10. The reactor of claim 5wherein said gas distribution plate comprises a counter electrode tosaid wafer support pedestal, said RF power source being connected acrosssaid wafer support pedestal and said gas distribution plate.
 11. Thereactor of claim 1 wherein said array of holes is arranged in parallelrows of said holes, said assembly further comprising: plural elongateslots in said back plate overlying and opening into the holes of thecorresponding rows of holes, each of said slots communicating betweensaid orifice and the holes in the corresponding row of holes.
 12. Thereactor of claim 11 wherein said assembly further comprises plural gasflow-controlling orifices corresponding to the plural rows of saidholes, each of said slots communicating between a corresponding one ofsaid orifices and the corresponding row of holes.
 13. The reactor ofclaim 11 wherein said orifice is not aligned with the annular gapsformed between said pucks and said holes.
 14. The reactor of claim 11wherein each of said elongate slots has a width less than the diameterof each of said holes, and wherein each of said pucks contacts a facingsurface of said front plate.
 15. The reactor of claim 8 wherein each ofsaid annular gas passages is divided into a pair of partial arcuateannular sections.
 16. The reactor of claim 15 wherein said partialannular passages correspond to a coincidence between the annular gapformed by each puck within the corresponding hole and the correspondingslot.
 17. The reactor of claim 11 wherein said holes and pucks areformed in said front plate and said slots and orifice are formed in saidback plate.
 18. The reactor of claim 17 wherein said front and backplates comprise silicon carbide.
 19. The reactor of claim 18 wherein atleast one of said plates further comprises an enclosed planar layer ofgraphite extending parallel with a plane of said plate.
 20. The reactorof claim 17 wherein said front and back plates comprises anodizedaluminum.
 21. The reactor of claim 17 wherein said gas distributionplate comprises a ceiling electrode of said reactor, said reactorfurther comprising a second RF power source coupled to said gasdistribution plate.
 22. The reactor of claim 17 wherein said gasdistribution plate comprises a counter electrode to said wafer supportpedestal, said RF power source being connected across said wafer supportpedestal and said gas distribution plate.
 23. For installation at aceiling location of a plasma reactor for processing a semiconductorwafer and having a vacuum chamber side wall, a wafer support pedestalfor supporting said semiconductor wafer in said chamber, an RF powersource coupled to said wafer support pedestal and a process gas source:a gas distribution plate comprising: a front plate in said chamber and aback plate on an external side of said front plate, said gasdistribution plate comprising a gas manifold adjacent said back plate,said back and front plates bonded together and forming an assemblycomprising: (a) an array of holes through said front plate andcommunicating with said chamber, (b) at least one gas flow-controllingorifice through said back plate and communicating between said manifoldand at least one of said holes, said orifice having a diameter thatdetermines gas flow rate to the at least one hole; (c) an array of pucksat least generally congruent with said array of holes and disposedwithin respective ones of said holes to define annular gas passages forgas flow through said front plate into said chamber, each of saidannular gas passages being non-aligned with said orifice.
 24. Thereactor of claim 22 wherein said assembly further comprises an array ofgas flow-controlling orifices through said back plate communicatingbetween said manifold and corresponding ones of said array of holes. 25.The reactor of claim 24 wherein each of said orifices faces acorresponding one of said pucks, said assembly further comprising aplanar gap between each of said pucks a facing surface of said backplate, each planar gap communicating between the corresponding orificeand the corresponding annular gas passage.
 26. The reactor of claim 23wherein said assembly further comprises: one gas flow-controllingorifice through said back plate for each predetermined group ofneighboring holes, said orifice generally facing a center one of theholes of the corresponding group of neighboring holes; internal gaspassages connecting the center one of the holes with the other holes insaid group of neighboring holes.
 27. The reactor of claim 25 whereinsaid back plate comprises said array of holes and said front platecomprises said array of pucks.
 28. The reactor of claim 27 wherein saidfront and back plates comprises silicon carbide.
 29. The reactor ofclaim 28 wherein at least one of said plates further comprises anenclosed planar layer of graphite extending parallel with a plane ofsaid plate.
 30. The reactor of claim 27 wherein said front and backplates comprises anodized aluminum.
 31. The reactor of claim 23 whereinsaid array of holes is arranged in parallel rows of said holes, saidassembly further comprising: plural elongate slots in said back plateoverlying and opening into the holes of the corresponding rows of holes,each of said slots communicating between said orifice and the holes inthe corresponding row of holes.
 32. The reactor of claim 31 wherein saidassembly further comprises plural gas flow-controlling orificescorresponding to the plural rows of said holes, each of said slotscommunicating between a corresponding one of said orifices and thecorresponding row of holes.
 33. The reactor of claim 31 wherein saidorifice is not aligned with the annular gaps formed between said pucksand said holes.
 34. The reactor of claim 31 wherein each of saidelongate slots has a width less than the diameter of each of said holes,and wherein each of said pucks contacts a facing surface of said frontplate.
 35. The reactor of claim 34 wherein each of said annular gaspassages is divided into a pair of partial arcuate annular sections. 36.The reactor of claim 35 wherein said partial annular passages correspondto a coincidence between the annular gap formed by each puck within thecorresponding hole and the corresponding slot.
 37. The reactor of claim31 wherein said holes and pucks are formed in said front plate and saidslots and orifice are formed in said back plate.
 38. The reactor ofclaim 37 wherein said front and back plates comprise silicon carbide.39. The reactor of claim 38 wherein at least one of said plates furthercomprises an enclosed planar layer of graphite extending parallel with aplane of said plate.
 40. The reactor of claim 31 wherein said front andback plates comprises anodized aluminum.
 41. The reactor of claim 23where the annular gas passages are circular.
 42. The reactor of claim 23wherein the annular gas passages are non-circular.
 43. A method offabricating a gas distribution plate for use in processing semiconductorwafers in a plasma reactor, the gas distribution plate comprisingsilicon carbide and having plural parallel slots communicating abovewith respective gas flow-controlling orifices and below with respectiverows of annular gas injection passages, said method comprising: formingparallel slots in one surface of a silicon carbide plate; filling saidslots with graphite inserts; depositing a layer of silicon carbide onsaid one surface of said silicon carbide plate; removing said graphiteinserts by heating said plate; drilling respective rows of annularopenings through the deposited layer of silicon carbide communicatingwith respective ones of said slots.
 44. A method of fabricating a gasdistribution plate for use in processing semiconductor wafers in aplasma reactor, the gas distribution plate comprising silicon carbideand having plural parallel slots communicating above with respective gasflow-controlling orifices and below with respective rows of annular gasinjection passages, said method comprising: forming parallel slots inone surface of a silicon carbide plate; covering said slots with siliconcarbide inserts; depositing a layer of silicon carbide on said onesurface of said silicon carbide plate; drilling respective rows ofannular openings through the deposited layer of silicon carbide andthrough said silicon carbide inserts, said annular openingscommunicating with respective ones of said slots.
 45. A plasma reactorfor processing a semiconductor wafer, said reactor comprising: a vacuumchamber side wall; a wafer support pedestal for supporting saidsemiconductor wafer in said chamber; an RF power source coupled to saidwafer support pedestal; a process gas source; a gas distribution plateat a ceiling location of said chamber, said gas distribution platecomprising: a front plate in said chamber and a back plate on anexternal side of said front plate, said gas distribution platecomprising a gas manifold adjacent said back plate, said back and frontplates bonded together and forming an assembly comprising: (a) an arrayof holes through said front plate and communicating with said chamber,(b) at least one gas flow-controlling orifice through said back plateand communicating between said manifold and at least one of said holes,said orifice having a diameter that determines gas flow rate to the atleast one hole; (c) flow diversion elements for transforming gas flowbetween said front back plates from stream patterns in said back plateto annular flow patterns in said front plate.
 46. The plasma reactor ofclaim 45 wherein: said stream patterns correspond to a first radius andsaid annular patterns correspond to a second radius larger than saidfirst radius; said flow diversion elements induce a rapid change of gasflow (a) from a vertical flow in each stream pattern (b) to a horizontalflow from said first radius to said second radius and (c) to a verticalflow each corresponding annular pattern.